I remember the recommendations on getting a new foot , and another to ensure that the collar was really tight so I checked the collar and managed to tighten it a bit more hoping that this would solve the issue taking bird shots in my back garden using my Manfrotto 322 pistol-head instead of the 3-way 029 head. Simply pressing the shutter while on the 322 head I could notice ever-slight movement in the D800. When using it in my backyard taking birds (which is generally in the shade) shooting at < 640asa and at least f8 I found that the ShutterSpeed is at least 1/125-1/250sec with the shots not as sharp as I would like them to be as I like to keep below 800asa for noise.

So I decided that using the shutter-cable from either hand could rule out both 'Camera-shake' and 'insecure tripod head' . I have found that this has helped in getting the shots sharper looking.

The gimbal head will make it a lot easier to shoot the birds - that's for sure.

At the slow shutter speeds you are using on the birds, you will need everything perfect to stop camera shake, but even then you will miss a lot of shots as the birds twitch. I make a point of NOT going below a ss of 1/500 and accept the iso will rise. The 80-400g is great wide open so in the garden where the light is often bad, I keep at f5.6 and a minimun of 1/500. Iso is ok up to 1600 and at a pinch up to 6400 if you dont have to crop too much and get the exposure right (ie not pushing shadows too much). There are many good NR plugins to reduce background noise from high iso.

The following link is typical of what I get with my garden set up (although done with a 500mm F4)

As soon as the light allows, I go to a minimum of 1/1600 to ensure sharp shots (avoiding birds twitching)

For birds, keep your ss up and you won't need to worry about camera shake. Another recommendation for your back garden shooting is to think about the background and getting near the birds. I bought a hide for around £80 which lets me get within 15-20ft and organised the sighting of it so I get good isolation of the bird to background - so it is easier to remove noise. Have a look at the first link above and what you can do with the background. High iso becomes less of a problem.